1. Field of the Invention
The invention relates to a method of generating a composite video signal formed from a foreground signal and a background signal in accordance with the luminance self-key mode, in which the key signal is derived from the luminance foreground signal in dependence upon a clipping level. The invention also relates to a circuit for performing the method.
2. Description of the Related Art
The television technique employs several methods of mixing a plurality of pictures or their corresponding video signals. A frequently used method is fading, or switching two picture signal sources, one of which supplies the foreground signal and the other supplies the background signal. By means of a control or key signal (usually derived from the foreground signal), these signals can be automatically switched within a picture period in dependence upon an adjustable color or an adjustable level occurring in the video signal, such that the background signal instead of the foreground signal is passed on or displayed within a picture when these adjustable values are reached, the fading in the case of the adjustable color also being denoted as chroma key and in the case of the adjustable level being denoted as luminance key. If the key signal is derived from the foreground signal in the luminance key mode, and this in dependence upon a clipping level and an adjustable gain, then it is referred to as the luminance self-key mode.
In the luminance key mode, the fading of the foreground signal FG and the background signal BG is effected in dependence upon an adjustable clipping level Clip by the luminance signal Y from a third picture source. In accordance with FIG. 1a, a key signal k, in accordance with FIG. 1b is derived from the luminance signal Y by means of the clipping level Clip, which fixes a 50% fading, said key signal serving for the fading of the foreground signal FG on the background signal BG, in accordance with FIG. 1c, so as to generate a composite output signal Vo. The foreground picture FG then appears in the displayed picture at those locations where the luminance signal Y clearly exceeds the clipping level Clip, and the background picture BG appears at those locations where the luminance signal clearly falls below the clipping level Clip.
The linear fading range, which is situated symmetrically around the clipping level Clip, may alternatively be fixed by means of a parameter Gain. Moreover, the fading range can also be determined by the minimum and maximum limits Yu and Yo. The fading is generally defined in accordance with the formula: EQU Vo=k*FG+(1-k)*BG (1)
in which Vo is the output signal composed of the foreground signal FG and the background signal BG produced during the fading process, and in which k is the key signal which is derived from the luminance information Y of the third source by processing with a clip and gain circuit. In this case, k has the value 1 when the signal Y is larger than Yo and, the value zero when the signal Y is smaller than Yu. In between, the key signal k assumes values of between 0 and 1. During this fading phase, only one fraction of the foreground signal FG is thus passed on, but is supplemented by a complementary fraction (1-k) of the background signal BG.
As described above, there are basically two modes of defining the variation of the key signal k. On the one hand, this may be effected with the aid of the clipping level Clip and/or the parameter Gain in which, however, in this case, the two limit parameters Yu and Yo, as from which the definitive foreground signal and background signal situations are reached, are influenced simultaneously. On the other hand, this may be effected by means of the two values Yu and Yo defining the proportional range. Then the lower clipping level Yu can be fixed independently of the upper level Yo, and vice versa. For the key signal k, the set-up EQU (Yo-Yu)/1=(Y-Yu)/k (2)
which puts given Y differences in a ratio with the associated k values, the following relation can be given: EQU k=(Y-Yu)/(Yo-Yu) at 0&lt;k&lt;1 (3)
A fading in accordance with equation (1) may be considered to be linear between the limit values, because there is a linear relation between the key signal k and the output signal Vo. This only applies when the foreground and the background signal sources are controlled by a third, separate signal source.
As already explained above, the key signal k is derived from the foreground signal FG in the luminance self-key mode, i.e., the controlling picture signal source is identical to the controlled picture signal source FG. As it were, the FG source fades itself in. The equation (1) for fading of the two sources is identical, but there is now a dual dependence on the same luminance signal. The key signal k has now originated from exactly the same foreground signal FG by linear processing, by which signal it should subsequently be multiplied. This "double multiplication" of the foreground signal FG yields a quadratic relation between the foreground signal FG and the product k*FG.
As a result of such a quadratic relation, the foreground luminance is underrated during the fading process. In accordance with FIG. 1d, a differently strong signal defect (black) is produced in the transition range between FG and BG, which defect leads to a black line along the key signal edge. This fading process takes place at each object edge of the picture contents of the foreground signal FG, at which edge a key signal change is generated.
EP 0 360 518 B1, corresponding to U.S. Pat. No. 4,947,255, discloses a device and a method in which this "double multiplication" can be avoided in the luminance self-key mode, in that instead of the foreground multiplication, the lower clipping level having the value of the complementary key signal is subtracted from the foreground signal by means of the key signal k. However, since the subtraction can only be usefully effected for the luminance signal in this case, the associated chrominance value is arbitrary. Thus, there is no definitive clipping level available for the chrominance value subtraction so that the currently known fading by multiplication by means of the key signal must be performed for the chrominance signal (cf. also the journal "JSMPTE", March, 1991, pp. 178 to 181, particularly page 181, paragraph 2).
For the sake of clarity, it will be assumed that the foreground signal represents light letters on a dark background. Due to the key process, the dark background should be separated from the letters and replaced by the background signal. The abovementioned process of fading from the foreground signal to the background signal, or conversely, takes place at all letter edges. This means that dark rims appear at all letter edges which become particularly disturbing when the new background for the light letters is also relatively light.